Since its discovery in 1899, it has been well known that via the Baeyer-Villiger reaction, ketones may be oxidized to esters by means of a percarboxylic acid. The mechanism of the reaction is such that a complex peroxidized intermediary is formed and subsequently rearranges itself according to an ionic process to yield an ester.
A particularly important application of the Baeyer-Villiger reaction is in the preparation of lactones from cyclic ketones. When the cyclic ketone cyclohexanone is oxidized by percarboxylic acid, .SIGMA.-caprolactone is formed.
In U.S. patent application Ser. No. 345,240, filed Feb. 3, 1982, direct use of crude percarboxylic acid solutions, such as crude perpropionic acid solutions for the oxidation of cyclohexanone is disclosed. These crude solutions are obtained according to the method described in U.S. Pat. No. 4,338,260 issued July 6, 1982 and of common assignee.
This method consists of causing the hydrogen peroxide to react with a water miscible carboxylic acid in the presence of a weak acid (boric acid) as a catalyst, while continuously eliminating the water from the reaction medium by azeotropic distillation using an inert organic solvent, such as dichloro-1,2-ethane. An improvement in this method, consisting of injecting water into the azeotropic distillation column, is described in French patent application No. 82 00407, dated Jan. 13, 1982. This improved method is described in Examples 2 and 3 below.
When the crude percarboxylic acid solutions thus obtained are used according to the teaching in U.S. Pat. application Ser. No. 345,240 for the oxidation of cyclohexanone, a molar ratio of cyclohexanone to percarboxylic acid between 1 and 5, and preferably between 1 and 1.5, is used. Under these conditions, yields of .SIGMA.-caprolactone are obtained in the order of 92% in relation to the peroxidic oxygen used.
The Kinetic study of the reaction between cyclohexanone and peracetic acid, undertaken by V. M. Vishnyakov et al. (JOURNAL OF APPLIED CHEMISTRY, USSR, Vol. 49, No. 9, p. 2035, 1976) showed that it was a bimolecular reaction of the first order with regard to each of the reagents. However, the prior art concerning the oxidation of cyclohexanone by peracids has always recommended using an excess of ketone in relation to the peracid. This excess of ketone is used as a dilutant for the reaction and avoids the annoying formation of peroxides which present a serious explosive danger.
Thus, molar ratios from 2 to 15 of cyclohexanone to peracid have been recommended whatever method of preparing percarboxylic acid is used:
aqueous solutions of peracid (Japanese patent application 45-15737/1970 and German Federal Republic patent No. 1,258,858),
peracid formed in situ by co-oxidation of cyclohexanone and acetaldehyde (K. Tanaka et al., Kogyo Kagaku Zasshi, 73, p. 943, 1970)
anhydrous organic solutions of peracid (P.S. Starcher, JACS, 80, p. 4079, 1958)
Although the use of a molar excess of cyclohexanone in relation to percarboxylic acid frequently has certain advantages from the point of view of ease and safety of operation, this method, on the other hand, has the disadvantage of requiring, after the oxidation reaction, the separation by distillation of residual cyclohexanone for its recycling.
In the case where the percarboxylic acid used is perpropionic acid, as in Federal Republic of Germany Patent No. 2,920,436, it is also necessary to effect the separation by distillation of cyclohexanone and propionic acid, each of these reagents being recycled at a different stage of the process. The existence of an azeotrope under reduced pressure between cyclohexanone and propionic acid makes this separation that much more difficult since they are compounds whose boiling points are very close. For example, under pressure of 100 mm mercury (13.3 kPa), the boiling points are as follows:
______________________________________ Propionic acid 86.degree. C. Cyclohexanone 93.degree. C. Azeotrope 93.degree. C. ______________________________________
On the other hand, it is not possible to effect this separation of propionic acid and cyclohexanone at ordinary pressure because the cyclohexanone condenses easily on itself under the effect of high temperature an also under the influence of various catalysts, especially acids, as shown by C. D. Hurd (JACS, 61, p. 3359, 1939). Among the products of condensation, there is notably formed a dimer, made up of a mixture of two isomers, cyclohexene-1-yl-2-cyclohexanone and cyclohexylidene-2-2cyclohexanone. Thus, at high temperatures, these by-products would then be found as impurities in .epsilon.-caprolactone.
While pursuing research on the applications of crude percarboxylic acid organic solutions obtained according to U.S. Pat. No. 4,338,260, the applicants discovered that, contrary to everything taught by the prior art, it is possible to effect the oxidation reaction of cyclohexanone by these crude peracid solutions by using an excess of peracid in relation to cyclohexanone, while conserving a high selectivity in relation to the ketone and peroxidic oxygen used and avoiding the formation of unsafe peroxides. This is possible because the peracid organic solution used by the applicants is practically anhydrous and devoid of all traces of a strong acid catalyst, such as sulfuric acid. Practically or substantially, anhydrous means that there is insufficient water present to adversely affect the reaction.
After the oxidation stage, the separations of the different constituents of the reaction mixture are made without any significant loss of residual peroxidic oxygen. As the engaged cyclohexanone is practically entirely consumed in the oxidation reaction, the top fraction obtained in the distillation of the crude oxidation products can be directly recycled at the stage of synthesis of the peracid solution. The bottom fraction, constituted essentially of .SIGMA.-caprolactone, is advantageously purified in a simple later distillation.
Thus, the advantage of the process of the invention is to provide very good yields of .SIGMA.-caprolactone with very low energy consumption in a simple and efficient manner.
The oxidation reaction of cyclohexanone by the crude peracid solution is achieved preferably at atmospheric pressure, but can also occur at lower or higher pressure. The temperature of the reaction is between about 20.degree. and 120.degree. C., and preferably between about 40.degree. and 80.degree. C.
The molar ratio of cyclohexanone used to percarboxylic acid is about 0.50 to 0.99, and preferably between about 0.75 and 0.90.
The reaction can be carried out batchwise or continuously. In the latter case, one or more reactors are fed by cascading cyclohexanone and crude percarboxylic acid solution simultaneously. The duration is between 30 min. and 4 hrs. depending on the temperature chosen for the reaction.
At the end of the oxidation, the reaction products are separated by distillation using the usual techniques. On the one hand, residual percarboxylic acid and the mixture of carboxylic acid and the entraining azeotropic solvent, and on the other hand the produced caprolactone are recovered. The distillations are advantageously carried out under reduced pressure to limit the losses of peroxidic oxygen and the thermal degradation of .SIGMA.-caprolactone. Preferably, evaporators currently in industrial use are employed, such as thin layered evaporators or film evaporators.
The following examples illustrate various aspects of the invention but the invention is not limited to them. In these examples, the amounts of .SIGMA.-caprolactone and cyclohexanone of the final solutions are determined by chromatography in the gaseous phase while the residual peroxidic oxygen is determined chemically.